1. Field of the Invention
The present invention relates to plasma etching systems. More specifically, the present invention relates to plasma sources used for plasma etching, chemical and physical vapor deposition, photo-resist stripping and other applications relating to semiconductor, flat panel display, printed circuit board and other fabrication processes.
2. Description of the Related Art
Plasma sources capable of generating uniform plasma over a wide area are needed for plasma etching, plasma enhanced chemical vapor deposition (CVD), physical vapor deposition, photoresist stripping and surface treatments for many applications. Illustrative applications include silicon and compound semiconductor fabrication, flat panel display fabrication including active matrix liquid crystal display, plasma display panels, field emission displays etc. Additional applications include hard disk drive head and media manufacturing, micro-electromechanical system manufacturing and printed wiring board fabrication.
A plasma source typically includes a radio frequency antenna, a dielectric tube and a volume of gas. An electric field from an impedance matched power supply is applied to the gas by the antenna through the dielectric tube. The application of the electric field to the gas generates two fields of interest with respect to plasma etching processes: a time varying electromagnetic field and an electrostatic capacitive field. The electromagnetic field strips free electrons from the gas in accordance with a first inductive coupling gas plasma technique. Ions generated by the application of the electric field to the gas are utilized in accordance with a second capacitive gas plasma technique. Free electrons gain energy from an electric field and generate ions by colliding with neutral gases. The inductive technique is known to be more efficient in producing ions than the capacitive coupling technique. A typical plasma etcher uses additional electric fields capacitively coupled to the substrate to increase ion energy.
Plasma sources using spiral antennas have been widely used for the above-described etching and processing applications. High plasma density and electromagnetic wave coupling uniformity are needed for such applications. When the conventional spiral antenna is expanded to cover a large area, the density of the electromagnetic energy decreases and most of the energy is coupled near the antenna. Efforts to increase the number of turns in the coil of the antenna have been limited by the self-resonance of the antenna. `Self-resonance` is an electrical characteristic of the antenna due to the inductive and capacitive effects of the antenna and limits the ability of the antenna to induce an electric field.
Also, obtaining uniform plasma was hard to achieve since the spiral antenna generates a highest electric field at the center of the source and a lower field toward the edge. Therefore, plasma has the highest density at the center and the density decreases sharply toward the edge due to dispersion. A magnetic field is often added at the plasma source wall to improve uniformity. However, even with the magnetic field, uniform plasma is hard to obtain.
Hence, there is a need in the art for a system or technique for generating a high density, uniform gas plasma affording a wide coverage area.